1. Field of the Invention
This invention relates in general to an improvement in a muffler provided with a catalyst, for an internal combustion engine, and more particularly to an improvement of an exhaust gas inlet pipe of such a muffler which pipe is provided therein with a venturi section for inducting secondary air under exhaust pulsation.
2. Description of the Prior Art
In connection with industrial trucks such as forklift trucks which are relatively small in size, a muffler for exhaust noise attenuation is frequently used in combination with a catalytic converter in order to reduce the length of an exhaust system. From this view point, a muffler provided therein with a catalyst has been proposed to achieve both noise attenuation and catalytic purification of exhaust gas. The muffler has an exhaust gas inlet pipe through which exhaust gas discharged from an internal combustion engine is fed into the muffler. The exhaust gas inlet pipe is provided with a venturi section through which atmospheric air as a secondary air is inducted into the muffler under the action of exhaust pulsation.
However, in such a muffler configuration, the exhaust gas inlet pipe is considerably short so that the distance between the venturi section and the rear open end of the inlet pipe is very small. Consequently, reflected wave of exhaust pulsation in such a short exhaust gas inlet pipe acts as interference wave on pulsation vacuum at the venturi section without being attenuated, thereby reducing the amount of secondary air to be inducted through the venturi section. This is caused even in a high frequency range of exhaust pulsation.
A problem of secondary air amount reduction is conspicuous particularly in the high frequency range of exhaust pulsation or in a high engine speed range. This is because, in addition to the above, atmospheric air or secondary air is naturally difficult to be inducted in such a range for the reason that the atmospheric air having a considerable inertia cannot follow momentary exhaust pulsation at the venturi section.
In order to solve the above-mentioned problem, it may be otherwise proposed to employ other secondary air induction device such as an air pump in place of a venturi system having the venturi section as in automotive vehicles. However, employing such secondary air induction device increases the number of parts while raising production cost of the exhaust system.